Recovery of materials from municipal and other solid wastes for recycling and reuse is of increasing importance in modern society, and increasing attention has been given to efforts to devise efficient, economical, and workable approaches to recovery of recyclable and reusable materials from the waste stream. In the past, the most commonly used approach has been source separation, in which the generator of the waste materials separates useable or recyclable materials from other wastes before any of the waste materials are removed from the generation site. The reusable or recyclable materials are then further separated to a degree determined by the use or uses to which those materials may be put, or by the willingness of the waste generator to engage in increasingly detailed separation efforts.
In the particular case of municipal wastes, source separation of wastes must be done by each waste generating household, and the willingness of the waste generators to participate in detailed separation programs is often a significant determining factor in the effectiveness of the separation, or recycling program. In even the most effective voluntary recycling programs, participation by household waste generators in source separation of wastes at all is not universal, and participation tends to drop as the complexity of separation increases. Problems also arise from errors made by municipal waste generators in identifying materials, resulting in commingling of recovered materials and reduction in the efficiency of separation. In response to the commingling problem, and often in an effort to increase participation, some recycling programs have provided for a combination of source separation and collection separation approaches. In the combined programs reusable materials are separated from other wastes and initially commingled, with additional separation performed as the wastes are collected from each generator. However, combined programs tend to slow waste collection and often require additional manpower, and any improvement in efficiency has been marginal at best.
Another approach that has been used in municipal recycling programs, either alone or in conjunction with separation by each waste generator, is post-collection separation, in which at least some part of the separation process is carried out after collection of wastes from individual households. Post-collection separation techniques range from simple and incomplete manual separation, based upon visual inspection of the waste stream and identification of recyclable materials, to more complex automatic systems utilizing techniques such as magnetic separation of ferrous metals, induction current separation of non-ferrous metals, and density separation in, e.g. water tanks. The effectiveness and efficiency of such separation techniques varies widely, and has proved to be only partially successful as an overall approach.
For some uses of recovered materials the particular composition of the materials is not as significant as uniformity of density and/or of particle size. For example, in the production of composite materials including recovered waste as filler, the chemical composition of the filler material may be of little importance, while the size and mass of individual particles may be of significant importance. Conventional separation approaches, such as those identified above, are based on separation by material type or chemical composition, and do not address density separation.
There remains a need in the field for a broadly applicable, effective, and economically efficient method of separating reusable and recyclable materials from solid wastes, especially but not necessarily limited to municipal wastes, and there likewise remains a need for apparatus for effectively and economically performing the separation method.